The present invention relates to offset printing presses and, particularly, to the electronic control of such presses.
Web offset printing presses have gained widespread acceptance by metropolitan daily as well as weekly newspapers. Such presses produce a quality black and white or color product at very high speeds. To maintain image quality, a number of printing functions must be controlled very precisely as the press is operating. These include the control of press speed, the control of color register, the control of ink flow and the control of dampening water.
In all printing processes there must be some way to separate the image area from the non-image area. This is done in letterpress printing by raising the image area above the non-image area and is termed "relief printing". The ink roller only touches the high part of the plate, which in turn, touches the paper to transfer the ink. In offset lithography, however, the separation is achieved chemically. The lithographic plate has a flat surface and the image area is made grease-receptive so that it will accept ink, and the non-image area is made water-receptive so it will repel ink when wet.
In a web offset printing press the lithographic plate is mounted to a rotating plate cylinder. The ink is injected onto an ink pickup roller and from there it is conveyed through a series of transfer rollers which spread the ink uniformly along their length and transfer the ink to the image areas of the rotating plate. Similarly, dampening water is applied to a fountain roller and is conveyed through one or more transfer rollers to the non-image areas of the rotating plate cylinder. The plate cylinder rotates in contact with a blanket cylinder which transfers the ink image from the plate cylinder to the moving paper web.
It is readily apparent that the amount of ink and dampening water supplied to the plate cylinder is directly proportional to the press speed. At higher press speeds the plate cylinder and blanket cylinder transfer ink and water to the paper web at a higher rate, and the inking and dampening systems must, therefore, supply more ink and water. It is also well known that this relationship is not linear and that the rate at which ink and dampening water is applied follows a complex rate curve which is unique to each press and may be unique to each run on a press. Not so apparent is the fact that the ink and water may be applied nonuniformly across the width of the ink pickup roller and the fountain roller in order to achieve uniform printing quality along the width of the web. If this is not done, there may be significant changes in the quality of the printed images across the width of the moving web.
Prior press control systems have provided limited control over the rate at which dampening water has been applied as a function of press speed. These systems pulse the nozzles on the spraybar on and off at one of a plurality of selectable pulse rates. The particular pulse rate selected is determined by the press speed. The particular pulse rates and selection points between pulse rates is preset to follow the dampening rate curve of the press as closely as possible. There is no means for easily changing these values or for providing a continuous range of pulse rates which closely follow the rate curve. In addition, while the amount of dampening water applied by the spraybar can be adjusted over the width thereof, this is a manual adjustment which may only be made locally at a spraybar controller. Thus, if inconsistencies in print quality are observed over the width of the image, manual adjustments to the circuitry must be made at a local control panel.